Microwave power rectifiers



May 20, 1969 c. e. CRUMLY 3,445,743

MICROWAVE POWER RECTIFIERS Filed March 16, 1967 inventor Charles B. Crumly,

\ Attorney United States Patent Oifice 3,445,748 Patented May 20, 1969 US. Cl. 321-8 12 Claims ABSTRACT OF THE DISCLOSURE Microwave energy fed to a bi-filar helix coupler is transferred thereby to an electron beam in the form of a positive synchronous wave. The modulated beam is then received by a depressed collector to which a load is connected. The apparatus functions as a rectifier of microwave power.

Introduction The present application relates to power rectifiers. More particularly, it pertains to apparatus for receiving and rectifying high-power microwave energy.

Various proposals have been advanced for delivering large quantities of power from a source to a load by means of microwave transmission. For example, where it is necessary to deliver power to a moving element of a machine or the like it may be difficult or impossible to use fixed connectors and the nature of the apparatus may be such as to preclude the use of such conventional expedients as slip rings or commutators together with brushes.

In a more exotic example, it has been proposed to transmit power to satellites, space vehicles and aircraft such as helicopters in the form of a concentrated beam of microwave energy. While numerous devices are known for generating, radiating and receiving high-power microwaves, great difiiculty is encountered at the receiving end in converting the received energy to a form convenient for use in energizing the ultimately driven equipment. Typically, the received microwave energy must be converted to direct current. In communication systems where the received power level need be only of the order of microwatts, or at most milliwatts, little difficulty is encountered with systems for rectifying intelligence signal information. However, when the microwave power levels involved are in terms of kilowatts, for example, conventional rectification techniques often become unwieldly or completely unusable because of their comparatively poor efficiencies and consequent power dissipation in the form of heat.

It is, therefore, a general object of the present invention to provide a microwave power rectifier overcoming the aforenoted disadvantages of prior rectifier apparatus.

Another object of the present invention is to provide a microwave power rectifier characterized by simplicity of operation while capable of obtaining a high level of eificiency.

A further object of the present invention is to provide a microwave power rectifier of a kind enabling substantial freedom of design and consequent flexibility of application.

A microwave power rectifier constructed in accordance with the present invention includes means for developing and projecting a beam of electrons along a predetermined path together with means for creating a magnetic field directed along that path and having a strength establishing a condition of cyclotron resonance for the electrons at a predetermined frequency. An electron coupler is disposed along the path and is responsive to signals from a source of microwave energy for selectively interacting only with the positive synchronous wave on the beam. Disposed across the beam path downstream of the coupler are means for collecting the electrons. Finally, a load is coupled to the collecting means.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements and in which:

FIGURE 1 is a diagramatic cross-sectional view of one embodiment of a microwave power rectifier;

FIGURE 2 is a plot of load current against load voltage and is useful in explaining the characteristics of the device of FIGURE 1; and

FIGURE 3 is a diagrammatic cross-sectional view of a modification of an element included in the device of FIG- URE 1.

The device illustrated in FIGURE 1 includes an elongated evacuated envelope 10 having an axis of symmetry 11. For developing and projecting a beam of electrons along a path generally coincident with axis 11, an electron gun 12 is disposed in one large end portion of envelope 10. Electron gun 12 includes as its primary element 'a cathode 13 connected by a terminal post 14, projecting through the end wall of envelope 10, to a plane of reference potential here shown as ground. The electron gun also includes a heating element for the cathode but, being strictly conventional, it has been omitted from the drawing.

The developed streamof electrons preferably is caused to converge toward axis 11 in the downstream direction. While numerous beam-converging electron guns are known to the art, as here illustrated such convergence is initiated by forming cathode 13 in the shape of a partial section of a sphere so that in operation the electron stream 15 tends naturally to converge as it departs from cathode 13. Cathode 13 preferably is disposed inside the open end of a cylindrical heat shield 16 closed across its end behind the cathode by a wall 17 having an aperture through which terminal post 14 projects. Completing the electron gun assembly is an annular anode or accelerating electrode 18 disposed downstream from cathode 13'.

Disposed across the electron path in the opposite enlarged end portion of envelope 10 is a depressed collector 20 for collecting the electrons at the end of their travel. As shown herein, collector 20 is in the form of a generally cylindrical anode having its surface facing the oncoming electrons shaped in the form of an inverted cone 21. Projecting from the opposite surface 22 of cone 20 and through envelope 10 is a terminal post 23 connected to ground through a load resistor 24 so that the load is coupled to the collector.

In order to confine the electron beam during its pas sage from the cathode to the collector and also to establish for the electrons a condition of cyclotron resonance, a magnetic field is created and directed along the beam path. While this magnetic field may -be created by a single, properly-shaped or contoured electromagnetic solenoid encircling envelope 10 along its intermediate region or by a suitably shaped permanent-magnet structure, as shown in FIGURE 1 it takes the form of a cylindrical solenoid supported between pole-defining flux-conducting end caps 27 and 28. End caps 27, 28 are annular in form, having peripheral portions abutting the respective end faces of solenoid 26 and defining respective inner surfaces 29 and 29 serving as pole faces and shaped to lie closely adjacent to the external surface of envelope 10.

One of the circular pole faces thus is disposed just downstream from cathode 13 and the other is just upstream from collector 20. In practice, end caps 27 and 28 are each split along a diameter to enable mounting.

Disposed along beam path or axis 11 between cathode 13 and collector is an electron coupler 30 which responds to microwave signals from a source 31 and selectively interacts only with the positive synchronous wave on the electron beam. Source 31 typically is a receiving antenna. To the end of very efficiently delivering energy only to positive synchronous wave motion of the electrons, coupler 30 is typically in the form of a bifilar helix wound coaxially around beam path 11 and occupying the intermediate region between the electron gun and the collector regions. As such, couple 30 is described and claimed in Robert Adler Patent No. 3,296,543, issued Jan. 3, 1967 and assigned to the same assignee as the present application. The two helical windings of the bifilar coupler are in this case coupled across a coil 32 with source 31 being in turn coupled across balanced points on coil 32 selected to match the impedance of coupler 30 to source 31.

In order to cause the electrons to travel from cathode 13 to collector 20, both windings of coupler 30 are biased at a positive potential relative to ground. To maintain a more uniform positive potential along substantially the entire beam path, envelope 10 may be formed of or coated with a conductive material biased to a positive potential. As illustrated, a positive potential source 34 is connected between ground and anode 18 and also is electrically connected to a center tap on coil 32.

The positive potential established throughout the intermediate region of the device causes the electrons to flow from cathode 13 to collector 20. The longitudinal magnetic field created by solenoid 26 tends to confine the electrons into a narrow beam throughout their travel. Moreover, pole face 29 adjacent to cathode 13 tapers inwardy in the downstream direction so as to create a magnetic field pattern which supplements the cylindrical shape of the cathode in causing the electrons to be converged from the large surface area of the cathode into the more narrow beam entering coupler 30. Similarly at the other end of the device, pole face 29 tapers outwardly in the downstream direction so as to create a magnetic flux distribution in that region which permits the electron beam to expand or diverge prior to impinging upon collector 20.

Of course, the strength of the magnetic field created by solenoid 26 is adjusted by selecting the value of direct current conducted through the windings of the solenoid; for this purpose, a direct current source (not shown) is coupled across solenoid terminals 36, 37. As indicated earlier, the strength of the magnetic field establishes a condition of cyclotron resonance for the electrons in the beam. The application of signal energy in the form of a transverse electric field causes the electrons to arrange themselves on helical loci or orbits having a periodicity determined by the strength of the magnetic field developed by solenoid 26 in accordance with the well understood cyclotron relationship; the cyclotron frequency f in megacycles is equal to 2.8 H, where H is the strength of the field in gauss. The number n of cyclotron orbits per unit length is expressed by the relationship where u is the average electron velocity in the axial direction.

An electron beam so formed and traveling through the longitudinal magnetic field is capable of several different modes of electron motion, one of which is termed the positive synchronous wave. Such a beam sometimes is described as exhibiting the positive synchronous wave and signal-energy created fields on an appropriate electron coupler are said to interact with or couple to that positive synchronous wave. Stated another way and applied specifically, coupler 30 serves as an input element and the signals from source 31 applied to coupler 30 create electric fields transverse to the beam path. These fields in this case initiate the development of a positive syncronous wave and cause its amplitude to grow along the length of the coupler. As so modulated, the beam assumes a helical shape and there is no overall rotation of the beam. The signal fields developed by the coupler, in this p0sitive-synchronous-wave mode, cause pure transverse motion of the beam; all electrons in the beam move axially at the same velocity.

To the end of achieving such interaction only with the positive synchronous wave, couple 30 is circularly polarized by cause it to have a twist in the same direction as the direction of twist of the helical electron arrangement caused by the magnetic field. The pitch of coupler 30 is assigned a value the same as that of the synchronouswave electron-helix pitch or pattern. This relationship is expressed as n f/u where n is the number of coupler helix turns per unit length and 1'- is the signal-interaction frequency (frequency of the signal from source 31). From Equation 1, the relationship of Equation 2 may be expressed in terms of the cyclotron frequency as follows:

"="c f f In coupler 30 as thus constructed, the electron velocity is such that a given electron passes one complete turn of the helical windings of coupler 30 for each signal cycle. The single electron efiectively is under the influence of a DC. field which causes the electron to drift at right angles to that field as well as to the axial magnetic field. Because of the circular symmetry of bifilar helix 30, corresponding forces are exerted on all electrons and the direction in which any given electron moves depends on its phase at entry into the coupler section. Consequently, the incoming narrow beam is spread into a corkscrew of growing diameter. The corkscrew pattern has the same direction of twist as the bifilar windings of coupler 30, and in passing through the coupler structure the electron pattern induces current in the coupler at the signal frequency. The phase of this induced current is such as to constitute a positive conductance load on the coupler. As a result, the device of FIGURE 1 constitutes a unidirectional signal energy translator in which the energy is carried on and stored by the electron beam in the form of the positive synchronous wave.

By utilizing the coupler to achieve pure monoenergetic positive synchronous wave development, the beam electrons are accelerated uniformly with a minimum of velocity spread. Axial velocity modulation at the second harmonic of the input signal is avoided by virtue of the circular polarization. Since the twist of the coupler renders it inherently circularly polarized, it discriminates against and prevents the development of the negative synchronous wave. The latter is a wave having a twist in the opposite sense and would result in a deceleration of the beam electrons with an increase in the power carried on the coupler. With the positive synchronous wave, however, interaction between the external circuit composed of the coupler structure and the synchronous wave on the beam is passive so that a complete transfer of coupler power to the synchronous wave may be accomplished with sufiicient length of the coupler.

It is to be noted that the coupler operates as a socalled lumped device as opposed to the operation of a traveling-wave coupler. That is, in coupler 30 the instantaneous signal potential along either of the bifilar windings is essentially the same throughout the length of the coupler. Where in a particular tube the coupler is of sufficient length that there is significant delay between the signals at the upstream and downstream ends, lumped operation preferably is enforced by electrically shorting together opposite ends of each of the bifilar windings.

The apparatus of FIGURE 1 permits wide flexibility of design. Since the signal from source 31 may be confined to a single carrier frequency of extremely small bandwidth, coil 32 is selected to tune the inherent capacitance of the coupler to resonance at the signal frequency with the circuit exhibiting a high Q corresponding to the narrow bandwidth. Similarly, the transformation properties of coil 32 enable source 31 to be accurately impedance matched to the electron beam through coupler 30. The magnetic field strength also is an uncritical parameter in the design of the device and thus can be somewhat freely chosen. A high magnetic field strength is desirable from the standpoint of efiiciency and power handling capability for given transverse dimensions; on the other hand, low values of magnetic field are, of course, desirable from the standpoint of magnet design, magnet materials, size and weight. As indicated by Equation 3, the magnetic field strength, and hence the cyclotron frequency, may be chosen to have any of a variety of values relative to the signal frequency.

The length of coupler 30 is not determined by bandwidth consideration as is usual in the typical low-level signal amplifier utilizing such couplers, although in a given system a requisite bandwidth might be specified in order to achieve a desired transient response. In general, however, there is no criticality on the coupler length. Nevertheless, if made very short there is the possibility of interaction with cyclotron and negative-synchronous waves; these are to be avoided because of their deteriorating effect on performance. At the same time, the coupler should be sufliciently long that any transient conditions excited on the beam at its entrance into the coupler are negligible compared to the desired positive synchronous-wave amplitude at the output end of the coupler. Where undesirable transients are encountered, their amplitude may be reduced by appropriately shaping the entrance region of the coupler; that is, the first few turns may slightly decrease in diameter in the downstream direction to define an outwardly flared entryway so that the signal field from the coupler is applied to the beam with gradually increasing strength in the downstream direction.

At the operating frequency, coupler 30 presents a matched termination to the signal source which is independent of power level. As a result, substantially all incident power is absorbed into the beam where it is manifested in an increased kinetic energy of the electrons. That is, the input microwave energy is stored in the electron beam, the altered electron positions represent a source of potential energy, and it is the function of collector 20 and load 24 to derive the stored energy in the form of direct current.

The electrons accepted by depressed collector 20 flow through load 24, rendering the end of load 24 connected to terminal 23 negative in relation to ground and also negative relative to the potential of the beam as it enters the collector region. It is this negative potential that effects depressed collector operation and causes the electrons to be decelerated and yield up power to the collector circuit. Terminal 23 represents to load 24 a source of constant current (essentially equal to the beam current) which is independent of load resistance up to a critical point where maximum DC. power is extracted. Beyond that point, the current rapidly drops to zero because the collector begins to reflect the beam back toward the coupler interaction area. This critical point is a direct function of the microwave power input.

Such behavior is depicted in FIGURE 2 where load current I is plotted as the ordinate and load voltage V as the abscissa. The point I represents the condition of zero input signal power so that the load current equals the DC. beam current and, because of the depressed mode of collector operation, the load voltage is zero corresponding to a load resistance of zero. Curve 40 represents the load current characteristic for a first signal power input. Consequently, for maximum power output load 24 is selected to present a DC. resistance defining a load line R which crosses curve 40 just before the point where the curve begins to drop in value. Similarly, curve 41 represents the operating characteristic with a higher signal power input; in that case, load 24 is selected to have a load resistance defined by load line R For a still higher microwave input the operating characteristic is defined by curve 42 for which the load resistance is selected to define load line R In any case, when the load resistance is too low for the power being fed into the coupler, so that the load line falls to the left of the knee in the curve in FIGURE 2, excess power is undesirably dissipated in collector 20.

On the other hand, if the load resistance is too high, the beam is at least partially reflected with the result that power is undesirably dissipated in one or more of the high-potential electrodes upstream from collector 20. In an exemplary application of the device, a system of storage batteries are connected to serve as load 24 with the operating voltage of the battery system being chosen just below the knee of the curve appropriate to the microwave power level applied to the input of coupler 30; alternatively, the input power to the device is selected relative to the desired load system voltage.

The interesting applications of the disclosed device are in so-called large-signal operation, as compared to the typical small-signal operation encountered in intelligence communications systems. With coupler 30 of uniform pitch and diameter as shown in FIGURE 1, the power handling capability of the device is limited by interception of the beam with the output end of the coupler and by loss of synchronism due to increased axial electron velocity near the output end of the coupler. These limitations may be extended by substituting in the device of FIGURE 1 the modified coupler 30a shown in FIGURE 3. Specifically, both the pitch and the diameter of the bifilar windings are caused to gradually increase throughout the length of the coupler in the downstream direction.

To illustrate one particular form of the device of FIG- URE 1, it may be assumed that the cyclotron frequency f is chosen to be equal to the signal frequency f with both frequencies being 3000 megahertz. The DC. beam power is assigned a value of 10 kilowatts and its perveance is 2 microper-vs. This results in a beam current of 1.32 amperes at a beam voltage of 7,580 volts. Coupler 30 at that voltage has a pitch of 1.48 turns per inch. The Brillouin beam density at the strength of magnetic field (1,071 gauss) necessary for a cyclotron frequency of 3000 megahertz, and at the designed beam voltage is 46.2 amperes/ centimeter? For typical operation at 70 percent of theoretical Brillouin current, the actual density is 32.3 amperes/centimeter To achieve this density, the amount of beam convergence from cathode 13 is only about 6.5 to 1, utilizing a dispenser cathode operating at 5.0 amperes/ centimeter? Based on the above operating current density and total beam current, the calculated beam diameter is 0.090 inch. By assigning an inner diameter to coupler 30 of 0.180 inch (twice the beam diameter), it can be shown that the maxi-mum power capable of being carried by the positive synchronous wave is 6940 watts.

Constructing the elements of twisted bifilar coupler 30 of flat, tape-like windings which together fill one-half of the axial space parallel to the beam, the required width of each winding in the axial direction is 0,169 inch. This construction results in a space harmonic reduction factor of 0.90. Assigning a length to the coupler of 3.0 inches, the beam loading resistance becomes 149 ohms; it may be noted that the beam loading resistance is inversely proportional to the square of the coupler length.

Of course, with the power levels involved it is very important to keep the coupler circuit losses to a minimum, since each 0.1 db loss of microwave power represents a loss of about 2 percent in efficiency. By virtue of the fact that the couplers need not exhibit a broad bandwidth, the

coupler circuit losses may be held to less than 0.2 db, representing only a 4.7 percent power loss.

In practical operation, the amount of power dissipated in depressed collector 20 is of the order of percent of the original beam power. Thus, with a power output of 6940 watts, approximately 500 watts are dissipated in the collector and 326 watts in coupler 30.

In terms of overall efliciency, then, the power output plus the collector dissipation and circuit losses total, in this example, 8096 watts. That represents a maximum efficiency of a very respectable 85.6 percent. In the overall sense, a small further reduction in that efficiency results from the power delivered to the heater for cathode 13. Additional efliciency reduction may result in given applications from unavoidable stray electron interception by the different electrodes in the device. Of course, interception on coupler 30 is to be avoided since an interception of only 1 percent of the beam current at full voltage in this example would result in a dissipation on the coupler of 100 watts or more and this would result in a corresponding reduction in the overall efficiency of about 1 percent.

As can be seen from this illustrative example, a device utilizing a coupler only 3 inches long is capable of delivering rectified power to a load in a magnitude measured in kilowatts. At the same time, high efficiencies of the ortier of 80 percent are obtainable. Consequently, the disclosed devices are particularly suitable for microwave power transmission and rectification. At the same time, the devices are simple in structural design and feature the use of individual parts or components of a straightforward nature.

While particular embodiments of the invention have shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects.

I claim:

1. A microwave power rectifier comprising:

means for developing and projecting a beam of electrons along a predetermined path;

means for creating a magnetic field directed along said path and having a strength establishing cyclotron resonance of said electrons at a predetermined frequency;

an electron coupler disposed along said path and responsive to microwave signals for selectively interacting only with a positive synchronous wave on said beam;

means comprising a collector having a potential depressed substantially relative to the potential of said coupler and disposed across said path downstream of said coupler for collecting said electron;

and a load coupled to said collecting means.

2. A rectifier as defined in claim 1 in which said developing means creates a stream of electrons which converge toward said coupler.

3. A rectifier as defined in claim 1 in which said coupler is circularly symmetrical about an axis defined by said beam path and has a diameter increasing in the downstream direction in an amount generally corresponding to increasing growth of said synchronous wave.

4. A rectifier as defined in claim 1 in which said c0upler is a bifilar helix encircling said beam path.

5. A rectifier as defined in claim 4 in which said helix has a pitch which increases in the downstream direction generally in correspondence with an increase along the helix of the energy in said synchronous wave.

6. A rectifier as defined in claim 4 in which said electrons follow helical orbits in consequence of said condition of cyclotron resonance and in which said bifilar helix has a twist of the same sense as that of said orbits and a pitch such that where n is the number of helix turns per unit length, n is the number of electron helical orbits per unit length, 1 is the nominal frequency of said microwave signals, and f is the cyclotron resonance frequency.

7. A rectifier as defined in claim 1 in which said developing means is coupled to a plane of reference potential and said load is coupled between said collecting means and said plane of reference potential.

8. A rectifier as defined in claim 1 in which the developing means is coupled to a plane of reference potential and said coupler is biased to a potential substantially positive relative to said plane.

9. A rectifier as defined in claim 1 in which said creating means includes pole-defining elements disposed adjacent to said path at opposite ends of said electron coupler.

10. A rectifier as defined in claim 1 in which said coupler is biased to a predetermined positive potential and said load is coupled between said collecting means and a potential plane substantially negative relative to said predetermined positive potential.

11. A rectifier as defined in claim 1 which further includes means matching the impedance of said source to the impedance of said coupler.

12. A rectifier as defined in claim 1 in which the impedance of said load is of a value selected to extract substantially a maximum level of power from said collecting 6/1965 Dain et al. 330 4.7 1/1967 Adler 3304.7

JOHN F. COUCH, Primary Examiner. W. H. BEHA, JR., Assistant Examiner.

U.S. Cl. X.R. 

